1. Field of the Invention
This invention relates generally to an all-fiber, narrowband optical filter and, more particularly, to a dynamic all-fiber, loop-mirror optical filter that provides filtering by a pump-induced saturable gain grating or a saturable absorber grating.
2. Discussion of the Related Art
Optical filters are an integral part of most optical systems. All-fiber, optically-tunable filters, including narrowband optical filters are beneficial in many of these optical systems, for example, in optical spectrum analyzers, RF signal processing in analog optical communications links, subcarrier multiplexing and demultiplexing in optical networks, subcarrier removal in digital optical networks where subcarriers are used for signaling, subcarrier signal processing in subcarrier-multiplexed optical fiber communications, ASE-noise removal, and channel routing and monitoring in wavelength-division-multiplexed (WDM) communication networks. All-fiber filters are important to reduce insertion losses in the optical system, and reduce the size and weight of the system.
Fiber Fabry-Perot filters, thin-film dielectric interference filters, conventional fiber Bragg gratings, acousto-optic tunable filters, and arrayed-waveguide grating routers represent the current technology available to provide optical filtering. These techniques and approaches, however, all suffer from one or more drawbacks, including the difficulty to provide bandwidths less than 1 GHz, operation only at fixed wavelengths or over a limited tuning range, requirements for mechanical or temperature tuning, inherent temperature sensitivity, non-fiber design, and finite free spectral range or periodicity. These various optical filtering techniques, as well as other known optical filtering techniques, are discussed and compared in the article by D. Sadot and E. Boimovich, xe2x80x9cTunable Optical Filters For Dense WDM Networks,xe2x80x9d IEEE Communications Magazine, December 1998, pgs. 50-55.
Investigations have been previously performed in the art using low-concentration erbium-doped fibers as a saturable medium to attempt to develop bandpass optical filters based on both saturable gain and saturable absorber gratings. See for example, S. J. Frisken, xe2x80x9cTransient Bragg Reflection Gratings in Erbium-Doped Fiber Amplifiers,xe2x80x9d Optics Letters, Vol. 17, pp. 1776-1778, Dec. 15, 1992; B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, xe2x80x9cNonlinear four-wave mixing in erbium-doped fiber amplifiers,xe2x80x9d Electronics Letters, Vol. 29, pp. 1858-1859, Oct. 14, 1993; and B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, xe2x80x9cNonlinear wave mixing and induced gratings in erbium-doped fiber amplifiers,xe2x80x9d Optics Letters, Vol. 18, pp. 2108-2110, Dec. 15, 1993. However, the results proposed in these papers are limited, and complicated system configurations prevent their practical application.
The effectiveness of bandpass filters based on pump-induced saturable absorber gratings has been demonstrated for laser linewidth narrowing and a theory to explain its effect has been developed. See, for example, M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, xe2x80x9cNarrow-linewidth, singlemode erbium-doped fiber laser with intracavity wave mixing in saturable absorber,xe2x80x9d Electronics Letters, vol. 30, pp. 648-649, Apr. 14, 1994; M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, xe2x80x9cLinewidth-narrowing mechanism in lasers by nonlinear wave mixing,xe2x80x9d Optics Letters, vol. 19, pp. 1406-1408, Sep. 15, 1994; Y. Cheng, J. T. Kringlebotn, W. H. Loh, R. I. Laming, and D. N. Payne, xe2x80x9cStable single-frequency traveling-wave fiber loop laser with integral saturable-absorber-based tracking narrow-band filter,xe2x80x9d Optics Letters, vol. 20, pp. 875-877, Apr. 15, 1995; and M. Horowitz, R. Daisy, and B. Fischer, xe2x80x9cFiltering behavior of a self-induced three-mirror cavity formed by intracavity wave mixing in a saturable absorber,xe2x80x9d Optics Letters, vol. 21, pp. 299-301, Feb. 15, 1996.
What is needed is an all-fiber, optically-tunable, narrowband optical filter that does not suffer from the drawbacks mentioned above. It is therefore an object of the present invention to provide such a filter.
In accordance with the teachings of the present invention, an all-fiber, optically-tunable narrowband optical filter is disclosed that makes use of a loop-mirror. The optical filter includes a coupler having two input fiber ports and two output fiber ports, where the output ports are connected together to form the loop-mirror. An optical saturable medium is positioned in the loop-mirror, which is either a saturable absorber medium or a saturable gain medium. A potentially broadband optical signal light to be filtered and a pump light are applied to the input ports of the coupler. The counter-propagating light waves in the loop generated by both the input signal light and the pump light create a signal standing wave interference pattern and a pump standing wave interference pattern in the saturable medium. Whether the input signal and the pump are applied to the same input port or different input ports determines whether the two standing wave interference patterns are in phase with each other or xcfx80 radians out of phase with each other, as set by the operation of the loop-mirror.
If the saturable medium is a saturable gain medium, then the peaks, or nodes, of the pump interference pattern bleach the gain of the saturable medium at the peaks, preventing amplification of the input signal at those locations. Likewise, if the saturable medium is a saturable absorber medium, the peaks of the pump interference pattern bleach the absorption of the saturable medium at the peaks, preventing absorption of the input signal at those locations. Therefore, depending on whether the saturable medium is a saturable absorber medium or a saturable gain medium, and whether the input signal interference pattern and the pump interference pattern are in phase or xcfx80 radians out of phase with each other, the filter is a bandpass filter or a notch filter.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.